The invention relates to a device for actuating a security device, preferably for securing a motor vehicle against unauthorized use, in which a control unit has means for transmitting a first coded electromagnetic signal (stimulus signal), in which a portable transmitter (radio key) has means for receiving the stimulus signal and for transmitting a second coded signal (enable signal), and in which the control unit is connected to the security device and actuates the latter if the enable signal is received and recognized.
Such radio keys are used today for unlocking the doors of motor vehicles without contact, for example. They are known from WO 92/18732, for example.
If the steady-state transmission and reception frequencies for such conventional systems are known, relatively simple transceivers can also forward the stimulus signal over relatively long distances from the vehicle to the authorized user and hence stimulate a key. If appropriate transmitters and receivers are also used for transmitting back the response signal, the response signal can also be traced back to the vehicle and used for unauthorized access to the vehicle.